1. Field of the Invention
This invention relates to a digital communications system and particular to a frequency division multiplex access (FDMA) communications system.
2. Description of the Related Art
It is known in digital communications systems to utilize digital Time Division Multiplex Access (TDMA) in which, for example, emergency broadcasts adopt a time slot already in use, but this results in data from a calling transmitter previously using the time slots adopted by the emergency broadcast being lost.
In a Digital Interchange of Information and Signaling (DIIS) protocol, which is currently being developed by the European Technical Standards Institute (ETSI), a so-called “super frame” has been devised in which four groups of sixteen slots of payload comprising voice or data is interleaved by three groups of two slots each of signaling frames known as Transmit Interrupt (TI) frames. The TI frame is used so that a transmitting digital communicating device, for example a radio or a base station, periodically returns to a receive mode during the TI frame in order to enable other radios, preferably mobile radios, the opportunity of being able to signal/interrupt the transmitting radio, for example for power control, emergency pre-empt, etc. Although TI frames occur on a regular basis so that a radio wishing to interrupt knows when the TI frame will occur, interrupts are likely to occur infrequently.
It is also known in radio communications for there to be gaps in channel activity, for example between a calling radio receiving an acknowledgement from a called radio and the called radio being taken “off hook”, i.e. a user answering the call. There is a further gap when a call is in progress between the called radio being taken off hook and a payload being transmitted by the calling radio. Another gap occurs between a payload transmitted by the calling radio and a payload being transmitted by the called radio and also at the end of transmission by the called radio. The gap between transmission of payload between the calling and called radios is sometimes referred to as an “over” period.
In the prior art it is known to fill these gaps with periodic channel reservation transmissions to indicate that the channel is busy and to prevent third party radios not involved in the call from acquiring the channel for their own purposes. The result quite often is that there are substantial periods of time when radios involved in a call do not have payload to transmit, but at the same time third party radios not involved in the call are waiting to transmit but are locked out and prevented from making transmissions. In a practical example, consider a dispatcher in a taxi service located at a base station who is in communication with a mobile radio in a taxi but who lacks signaling communication from other taxis who may wish to use the communication channel. It is desirable that the dispatcher be able to receive information such as the location of taxis other than the one with whom he is communicating so that the location of those other taxis may be plotted on a display screen, the location being drive by, for example, using a Global Positioning Satellite (GPS) system.
Published U.S. Pat. No. 5,719,868 describes a TDMA slot assignment method. This patent deals with simplex radios operating in self regulating ad hoc configurations and being used to route packet data. A channel contains a cycling TDMA type of frame structure in which each frame consists of multiple slots. If there are ‘n’ radios, then it appears there must be ‘n’ frames in the cycle. Slot 0 in each frame is referred to as the broadcast slot. Each radio is assigned its own broadcast slot with which it can allocate itself channel resource and inform its neighbors. A radio allocates itself channel resource by assigning itself specific slots, however, not the broadcast slots. It appears that these slot allocations can either be on the current channel, i.e. the same channel as the broadcast slots, or an alternative channel. The arrangement of U.S. Pat. No. 5,719,868 appears to involve individual radios keeping track of which slots are allocated to other radios, so that when a given radio needs to transmit packet data it can allocate itself unused slots on a given channel, thereby not colliding with other transmissions.
In essence, the arrangement of U.S. Pat. No. 5,719,868 deals with allocating specific slots on a TDMA channel to specific radios so that they can allocate themselves channel resource, thereby minimizing the risks of collision.
U.S. Pat. No. 5,754,536 describes radios operating on a Trunked system, and traffic channels only being allocated for the duration of the speech frames. So whenever a radio detects the presence of speech, it informs the trunking system controller, which then allocates a traffic channel for the duration of the speech frame.
In essence, the arrangement of U.S. Pat. No. 5,754,536 deals with a trunking system controller allocating channel resource to radios only when there is speech to be transmitted.